This invention relates to an adjustment control circuit which controls adjustment of an input signal, and relates particularly to a color temperature switching circuit incorporated in a cut-off adjusting circuit and a drive adjusting circuit of an improved television receiver (TV) or display monitor.
In accordance with the development of large screen TVs, higher picture quality is necessary, and thus various kinds of adjusting circuits including the cut-off adjusting circuit and the drive adjusting circuit have been developed.
In adjusting the white-balance which is one of the most important fundamental TV characteristics, the cut-off adjusting circuit adjusts each of the DC levels of three primary color signals, and the drive adjusting circuit adjusts each of the amplitudes of said color signals. These are the most important fundamental circuits for establishing the accuracy of the white-balance.
FIG. 6 shows a block diagram of the conventional cut-off adjusting circuit and the drive adjusting circuit designed to receive a primary color signal.
In FIG. 6, terminal 41 is a primary color signal input terminal. Block 42 is a cut-off adjusting circuit which adjusts the DC level of the primary color signal at terminal 41. Block 43 is a cut-off control circuit by which a digital control signal is converted into an analog control signal for controlling the cut-off adjusting circuit 42. Block 45 is a drive adjusting circuit for adjusting the amplitude of the primary color signal that has a DC level that has been adjusted by cut-off adjusting circuit 42. Block 46 is a drive control circuit for converting a digital signal into an analog control signal for controlling the drive adjusting circuit 45. Block 44 is a micro-processor unit (MPU) for transmitting the digital control signals to the cut-off control circuit 43 and the drive control circuit 46. Terminal 47 is an output terminal for a primary color signal with a DC level adjusted by the cut-off adjusting circuit 42 and an amplitude adjusted by the drive adjusting circuit 45.
The operation of the cut-off adjusting circuit and drive adjusting circuit is explained below with reference to FIG. 7.
When a primary color signal at terminal 41 having a waveform as shown in FIG. 7(a) is input to cut-off adjusting circuit 42, the DC level of the primary color signal is adjusted to a level as shown in FIG. 7(b). The relative amplitude of the primary color signal exceeding a predetermined reference range determined by the amplitudes of the other primary color signals is adjusted by drive adjusting circuit 45 yielding a waveform as shown in FIG. 7(c).
This signal is then inverted and amplified by an inversion amplifier yielding an output of the primary color signal which is fed to the cathode of the Braun-tube of a TV (CRT) on which pictures are displayed. These signal adjustments have to be applied to each of the three primary color signals in order to accomplish the white-balance adjustment of a CRT.
The above-mentioned adjustments had been conducted within the inversion amplifier by using a set of manually adjusted variable resistors before the signal is outputted. The cut-off and drive adjustments of the primary color signal can now be performed before the primary color signal is fed to the inversion amplifier by applying control signals derived from a MPU as shown in the circuit in FIG. 6. The automatic white-balance adjustment circuit utilizing a MPU drastically reduces the man-power wasted in the white-balance adjustment.
In order to optimize the TV color reproduction of various video signal sources, switching of the color temperature according to the mode of the color temperature is performed by switching the cut-off and the drive voltage of the CRT. This can be performed easily by rewriting the MPU data.
However, in the above-mentioned control circuit, since the control of cut-off adjusting circuit and drive adjusting circuit and the control of the color-temperature mode switching are performed by using a common control circuit, the establishment of a linear relationship between the input digital control signal and the output analog control signal is essential to maintaining the resolution of the adjustment at a high level.
In actuality, however, the control range which maintains the linear relationship is limited. Moreover, while finer adjustments require a narrow controllable range, a wide control range has to be provided in order to adjust the color temperatures of video equipment including components having large variations such as a CRT. Thus, the controllable range can be made wider by adding a DC bias voltage to the analog voltage outputted from the adjust control circuit, and by adjusting the added DC bias voltage.
These operations can be illustrated by a stepped control characteristic curve obtained with a conventional cut-off control circuit or drive control circuit as shown in FIG. 8(a). The four steps in the control characteristic curves result from the application of four different added bias voltages including zero volts.
The control characteristic curves shown in FIGS. 8(a), 8(b) and 8(c) are produced by synthesizing a characteristic curve of the cut-off adjusting circuit 42 and a characteristic curve of the cut-off control circuit 43. The former curve is linear and the latter curve has overlapped steps.
The characteristic curves shown in FIGS. 8(a), 8(b) and 8(c) also illustrate the characteristics of the drive adjusting circuit. The characteristic curves are synthesized from a characteristic curve of the drive adjusting circuit 45 and a characteristic curve of the drive control circuit 46. The former curve is linear and the latter curve has overlapped steps.
However, in this case, it is very difficult to replace these four characteristic curves with a continuous single line containing no steps because of either the variations of the input/output conversion characteristics of the control circuits, or the variations of the bias voltages to be added.
If the bias voltage to be added is too high, the characteristics curve showing the control characteristics is shifted upward as shown in FIG. 8(c), producing regions in which necessary variations are unavailable. In order to avoid this situation, the bias voltages to be added have to be set at values less than the necessary values. Thus, a characteristic curve with overlapped steps as shown in FIG. 8(a) is produced.
Because of these steps, however, the variation would take a value of y' when the cut-off adjusting point is at point-a, while the variation would take a value of y when the cut-off adjust point is at point-b, even though the offset X step remains the same, thus producing a difference in the variations obtained in these two cases (see FIG. 8b).
Therefore, the value of off set has to be readjusted each time the mode switches (such as color temperature switching), so that mode switching is very difficult to perform because of the different off set values which have to be determined according to the cut-off adjusting data.